"The time when "EVERYONE" in Europe was White does not exist." @medievalpoc

Maybe this is a crazy question, but how did Europeans know what Africans looked like? I know that some of the paintings here are of North Africans/Middle Easterners, but others clearly depict people born south of the Sahara. I've heard of Prester John but I never imagined that medieval Europeans were aware that Prester John would have had brown skin. Am I missing something?